A technique for surveying subsea strata is to tow a number of hydrophones behind a boat over the area to be surveyed. The hydrophones are mounted in some geometric pattern. An echo-producing impulse, such as an explosive blast or the like, is generated in the vicinity of the hydrophones. The sound from the impulse means is echoed back from the seabottom and the underlying strata to the hydrophones. The outputs from the hydrophones are connected to recording equipment that records the outputs to indicate the profile or character of the area being surveyed.
A variety of cable suspension arrangements and hydrophones have been developed. An early attempt to position a string of hydrophones was to position the hydrophones on the surface of the water. Because of the surface noise generated by wave action and the noise generated by the slapping of the waves on the cable, it was determined that surface mounted hydrophones were unsatisfactory and that satisfactory results might be obtained by submerging the hydrophones below the surface of the water.
One underwater system mounts a number of hydrophones on a vertical cable which is supported on the upper end by a float and weighted on the bottom end with a weight. It was discovered that the drift of this hydrophone array caused by ocean currents produced mechanical vibrations on the supporting cable which, in turn, caused the hydrophone array to be subjected to unwanted spurious signals. This mechanical vibration is commonly referred to as "strumming".
Another technique is to utilize a nonfloatable support cable which is maintained in a substantially horizontal position beneath the surface of the water by the velocity of a towing ship. The hydrophones were mounted along the length of the cable. It was discovered that undesirable noise was produced in this system by the vertical motion of the hydrophones in the water which caused the hydrophones to be subjected to varying water pressure. The hydrophones were unable to distinguish between the varying pressure caused by this vertical movement and the varying pressure caused by the explosive blast impulse.
A number of other techniques were developed by the prior art in an attempt to overcome these problems. One approach was to utilize a buoyant cable in an attempt to suspend the hydrophones at a predetermined distance beneath the surface of the water. However, the movement of the ship caused the hydrophones to rise above the desired depth. Therefore, whenever it was desired to make a reading, the tow ship would have to stop and the cable and hydrophones allowed to reach the equilibrium state before a seismic reading could be made. The ship would then restart and move to the next position. This increased the time and hence the cost of surveying the ocean bottom. To reduce the time between seismic surveys, a paravane was used to maintain sonobuoys in a predetermined array during the movement of the ship. See U.S. Pat. No. 1,305,483 and U.S. Pat. No. 3,491,848. However, the paravane could maintain the array of sonobuoys in the predetermined shape and depth only when the towing vessel was moving at a relatively high rate of speed. The movement of the towing vessel at this high rate of speed again caused strumming of the supporting cable.
The system according to this invention overcomes these problems by providing means for maintaining sufficient tension on the suspension cable to maintain the hydrophones in a pair of straight lines that lie within the horizontal plane without requiring that the hydrophone array be towed at excessive speeds. This causes a reduction of the mechanical and fluid noise problems and permits the utilization of a much more efficient hydrophone array at a substantial reduction in cost.